Location and probability of shoal margin collapses in a sandy estuary

Abstract

Channel bank failure, and collapses of shoal margins and beaches due to flow slides, have been recorded in Dutch estuaries for the past 200 years but have hardly been recognized elsewhere. Current predictions lack forecasting capabilities, because they were validated and calibrated for historic data of cross-sections in specific systems, allowing local hindcast rather than location and probability forecasting. The objectives of this study were to investigate where on shoal margins collapses typically occur and what shoal margin collapse geometries and volumes are, such that we can predict their occurrence. We identified shoal margin collapses, generally completely submerged, from bathymetry data by analyzing digital elevation models of difference of the Western Scheldt for the period 1959-2015. We used the bathymetry data to determine the conditions for occurrence, specifically to obtain slope height and angle, and applied these variables in a shoal margin collapse predictor. We found 299 collapses along 300 km of shoal margin boundaries over 56 years, meaning that more than five collapses occur on average per year. The average shoal margin collapse body is well approximated by a 1/3 ellipsoid shape, covers on average an area of 34 000 m 2 and has an average volume of 100 000 m 3 . Shoal margin collapses occur mainly at locations where shoals take up a proportionally larger area than average in the cross-section of the entire estuary, and occur most frequently where lateral shoal margin displacement is low. A receiver operating characteristic curve shows that the forecasting method predicts the shoal margin collapse location well. We conclude that the locations of the shoal margin collapses are well predicted by the variation in conditions of the relative slope height and angle within the Western Scheldt, and likely locations are at laterally relatively stable shoal margins. This provides hypotheses aiding the recognition of these features in sandy estuaries worldwide.

Geometry and Topology of Estuary and Braided River Channel Networks Automatically Extracted From Topographic Data

“…Both the differential networks and the complete composite network for each data set are analyzed. Four data sets are used: A set of DEMs resulting from the morphodynamic modeling of a braided river (Schuurman et al, 2013), a lidar DEM of the Waimakariri River in New Zealand (Hicks et al, 2007), a set of DEMs from a morphodynamic model of estuary development (Braat et al, 2017), and a DEM of the partially dredged Western Scheldt estuary in the Netherlands (van Dijk et al, 2018. These data sets were chosen because they span a range of morphological conditions and variability in boundary conditions (i.e., coastal estuaries versus braided rivers).…”

Species selection and assessment of eco‐engineering effects of seedlings for biogeomorphological landscape experiments

“…These vegetation-driven changes in the landscape either favour or hinder new plant development in the next seeding cycle, and possibly facilitate other species that would otherwise not settle there. Figure 1 is an illustration of the biogeomorphological patterns that emerge when different vegetation species are combined in tidal experiments (Kleinhans et al, 2017a;Braat et al, 2018;Leuven et al, 2018) with scaling, sediment and vegetation treatment similar to that in van Dijk et al, (2013). Such experiments need to be developed in the future, but need a starting point of systematically analysed sensitivity of species to landscape experiment conditions and effects of the species on critical aspects of the hydromorphodynamics.…”

“…For rivers and estuaries these include salt marshes with, for example, Spartina species, mangrove forests including Avicennia species, riparian forests with, for example, Salicacea species and vegetated deltas with Nelumbo species. Figure 1 is an illustration of the biogeomorphological patterns that emerge when different vegetation species are combined in tidal experiments (Kleinhans et al, 2017a;Braat et al, 2018;Leuven et al, 2018) with scaling, sediment and vegetation treatment similar to that in van Dijk et al, (2013). The ecoengineering species in these environments have starkly different settling conditions, inundation and uprooting tolerance, rooting density and added bank strength and flow to higher bathymetry flow focusing/retardation (Corenblit et al, 2007) Saltmarsh Mean water level Soil enforcement Spartina, Juncus up to high tide substantial flow retardation (Schwarz et al, 2018) Freshwater floodplain Adjacent to channel Marsh-like effects for grasses; Phragmites, Typha up to highest flooded areas riparian type effects for trees resistance.…”

Watching the Beach Steadily Disappearing: The Evolution of Understanding of Retrogressive Breach Failures

“…Figure 3 shows the erosion and deposition pattern of a completely subaqueous flow slide event in a steep channel bank eroded by tidal currents near the Eastern Scheldt Barrier [7,8]. Van Dijk [9] assessed a large number of previously unnoticed subaqueous shoal margin collapses from historical bathymetry data of the Western Scheldt. De Bruin and Wilderom [10] collated historical information on the fate of polders related to dike failures and the struggle with the sea.…”